In the primary treatment of iron ore, it is well known to use magnetic separators with permanent magnets or electromagnetic systems for wet separation of grainy materials by the agency of their magnetic properties. Depending on the properties of the minerals under treatment and the process employed for separating grains of magnetic minerals, use is made of magnetic separators of various designs, said separators operating with appropriate values of magnetic forces acting on grains of magnetic minerals.
Most widely known in the art of ore concentration are drum-type magnetic separators with a magnetic field intensity of 60 to 160 kA/m (kiloamperes per meter).
A drum-type magnetic separator comprises a trough with a pulp feeding device and devices for discharging magnetic and nonmagnetic products, and a cylindrical drum made of a nonmagnetic material and mounted horizontally for rotation about the axis thereof. Said drum is partially situated in the trough and accommodates an magnetic system which is located inside the drum nearer the trough.
The pulp containing grains of magnetic and nonmagnetic minerals is delivered into the separator trough through the feeding device. As the pulp flows in the separator trough, the forces of the magnetic field created by the magnetic system attract grains of magnetic materials to the surface of the nonmagnetic drum located underneath the magnetic system. As the drum rotates, the magnetic mineral grains attracted by the magnetic field forces travel together with the drum toward the device for discharging the magnetic product, whereas the nonmagnetic material grains are carried by the pulp flow to the device for discharging the nonmagnetic product.
In such separators, the magnetic attraction forces should considerably exceed the dynamic forces of the pulp flow and the force of gravity of magnetic mineral grains. To this end, use is made of magnetic systems which create highly nonuniform magnetic fields of as high intensity as 60 to 160 kA/m in order to separate magnetic mineral grains from the pulp flow and retain them on the drum surface.
In said separators, because of strong magnetic interaction of magnetic mineral grains therebetween and with the magnetic field created by the magnetic system, lingering accumulations of magnetic mineral grains are formed on the drum surface, said accumulations entraining nonmagnetic mineral grains and concretions thereof with magnetic minerals.
Removal of entrained nonmagnetic mineral grains and their concretions from lingering accumulations of magnetic mineral grains is difficult and can be accomplished only by repeatedly cleaning the magnetic product, the effectiveness of operation of drum-type magnetic separators ever decreasing toward the final stages of treatment. Therefore, in spite of perfection of design, the drum-type magnetic separators under discussion do not provide for high selectivity of separating grainy materials by the agency of their magnetic properties, particularly in the processes of final treatment and recovery of pure iron concentrates from finely disintegrated and slimy ores.
Known in the art are magnetic separators designed for selective separation of magnetic mineral grains. Said separators depend for their operation on varying local concentration of magnetic mineral grains in flowing pulp under the action of a slightly nonuniform magnetic field of as low intensity as 2 to 10 kA/m.
With low intensity magnetic fields, the appropriate relationship of magnetic and dynamic forces produces a concentrated bed of movable magnetic mineral grains in the lower part of the pulp flow. Nonmagnetic mineral grains are readily washed out from said bed by water flows and, therefore, in this case the magnetic product can be recovered from the pulp without separating it by attraction to some surface by the agency of a strong magnetic field.
In low intensity electromagnetic separators use is made of electromagnetic systems whose attraction force is sufficient to change the line of travel of magnetic mineral grains, but is usually less than the force of gravity of the same.
Known in the art is an electromagnetic separator operating on the principle described above. This separator comprises a nonmagnetic housing made in the form of a vertical cylinder open on the top, a circular electromagnetic system located outside the housing at the bottom part thereof, a pulp feeding device with tangential branches located at the level of the electromagnetic system, a nonmagnetic product discharging device located in the top part of the housing, and magnetic product discharge branches located in the bottom part of the separator housing.
The pulp is delivered under pressure into the separator housing through the feeding device with the tangential branches so that the feed is given a circular rotary motion. When the magnetic field created by the electromagnetic system is applied to the pulp flow in the housing, it changes the line of travel of magnetic mineral grains so that instead of moving helically from bottom to top together with the main flow of the pulp they form a concentrated bed in the bottom part of the housing. As said bed is formed, grains of nonmagnetic minerals are washed out therefrom by upward flows of the pulp and carried into the nonmagnetic product discharging device. The grains of magnetic minerals accumulate in the bottom part of the separator housing and are discharged from the separator via the magnetic product discharge branch.
In the electromagnetic separator under discussion, the pulp feeding device, which is provided with tangential branches, does not cater for the required velocity of pulp rotary motion even at the maximum rate of pulp feed. This disadvantage results in failure to provide the relationship between the magnetic and dynamic forces appropriate to the magnetic field intensity required for the grains of magnetic minerals to form a bed of concentrated and yet movable material. Hence, the electromagnetic separator under discussion suffers from the disadvantage of low separation selectivity and low operating efficiency.
Also known in the art is a wet separation apparatus which consists of an open housing with a cylindrical upper part and a conical lower part.
The top or middle of the cylindrical part of the housing is in the form of a screen. Inside the housing is installed a shaft which drives disk-shaped distributing plates attached thereto. A row of vertical blades is attached to the edge of one of the distributing plates.
The distributing plates are installed one above the other so that they form an even, shallow slope. One or several plates have holes. The apparatus also comprises a pulp feeding device in the form of a circular pipe, a chute installed in the top part of the housing and designed for collecting and discharging fines, and a coarse product discharge branch located in the bottom part of the housing. A pipe with a circular channel for supplying wash water is provided in the conical bottom part of the apparatus.
The wet separation apparatus operates as follows:
The material to be separated according to size is fed in the form of pulp via the feeding device, which is constructed as a circular pipe, into the middle part of the apparatus housing where the pulp is uniformly distributed throughout the circumference with the aid of the distributing plates. The largest and, consequently, the heaviest grains form the lowermost layer. The fine material is carried by the pulp current into the top part of the apparatus where it is given a rotary motion about the vertical axis at a predetermined velocity by the rotating vertical blades attached to the edge of one of the distributing plates. This layer of the pulp moves at the appropriate angular velocity relative to the screen surface at rest and the whole mass of fine particles in the form of fluid pulp passes through the screen holes into the chute for collecting and discharging fines. The portion of the solid particles which does not pass through the screen holes settles into the bottom part of the housing where the coarse product is carried away through the discharge branch. The boundary separation size of the material under treatment can be regulated by setting the appropriate velocity of pulp rotation relative to the screen. The fine grains are removed from the bottom part of the apparatus by means of wash water supplied from the pipe which has a circular channel and is located in the conical bottom part of the housing. The wash water flows through the hole in the distributing plates and the gaps therebetween into the top part of the housing and makes for separating the coarse product from the fines.
The wet separation apparatus is designed for separating fines from a coarse material in a pulp flow, but it is not intended and cannot be used for wet separation of grainy materials according to their magnetic properties. Consequently, the efficiency of the apparatus in this type of work cannot be judged.
Yet it should be noted that the pulp flow in the middle and top parts of the housing is given high turbulence through agitation effected by the vertical blades due to which the pulp is strongly circulated in all directions and is thereby stirred both vertically and radially. Besides, the manner in which wash water is fed in the apparatus does not prevent fine mineral grains from getting through the gap between the distributing plate and the housing into the bottom part of the housing inasmuch as overpressure exists there and water delivered under pressure will not pass through said gap unless it is caused to flow in the required direction.
Known in the art is an electromagnetic separator for treating heavy ferromagnetic suspensions (U.S.S.R. Author's Certificate No. 543414, the year (1975), date of issue Mar. 15, 1977, comprising a cylindrical housing with a conical bottom, a circular electromagnetic system installed outside the housing, a cylindrical pulp feeding device installed inside the housing coaxially therewith, a paddle agitator located underneath the pulp feeding device, a light product discharging device located in the top portion of the housing, and a heavy product discharge branch located in the conical part of the separator housing. In this separator, pulp is fed through the feeding device into the housing where the paddle agitator imparts to it a circular motion at the velocity determined by the dynamic forces required for effective separation. The magnetic field created by the circular electromagnetic system changes the line of travel of the magnetic particles. By virtue of magnetic interaction at the appropriate relationship between magnetic and hydrodynamic forces a movable concentrated bed consisting mainly of magnetic mineral grains is formed in the bottom part of the separator housing. Nonmagnetic mineral grains and their concretions with magnetic minerals are easily washed out of this bed by uprising water currents of the pulp and are carried into the top part of the housing where they flow together with the pulp over the housing edge and get into the light product discharging device. The magnetic mineral grains contained in the concentrated movable bed in the bottom of the housing settle and are carried away via the heavy product discharge branch.
The separator under discussion is designed for specific gravity separation of a large-piece material in a concentrated movable bed of magnetic grains treated as a heavy suspension. Pieces of ore are delivered through the pulp feeding device into the movable concentrated bed formed in the separator. Light pieces of ore rise and are discharged by means of the light product discharging device, whereas heavy pieces of ore settle and are discharged via the heavy product discharge branch.
The separator for treating heavy ferromagnetic suspensions can also be used for separating grainy minerals by the agency of their magnetic properties. To create dynamic forces appropriate to the magnetic field, the pulp is given a rotary motion about the vertical axis by the use of the paddle agitator, whereby the proper relationship between magnetic and hydrodynamic forces required for producing a movable concentrated bed of magnetic mineral grains is readily obtained.
However, with this construction of the electromagnetic separator, the paddle agitator, which is located under the pulp feeding device, apart from imparting a major rotary motion to the pulp, also produces minor vertical pulp circulation above and below, which results in up and down stirring of grainy material. Besides, some of the pulp delivered through the feeding device flows directly into the bottom part of the housing where the magnetic product is discharged, the latter being contaminated. Furthermore, the pulp entrains air which moves in the housing upward at a high velocity, hampering the separation process.
Thus, the design of the separator under discussion fails to provide the hydrodynamic conditions of pulp flow required to prevent up and down stirring of the material and to allow removing nonmagnetic mineral grains and concretions from the magnetic product in the bottom part of the housing. This predetermines low selectivity of separation of grainy materials by the agency of their magnetic properties and, consequently, insufficient efficiency of the separator in this type of work.
Iron concentrates with low impurity content find wide and ever increasing use in various fields of the industry, for example, in nonblast-furnace production of iron, powder metallurgy, production of ferrite, making of catalysts, etc.
It is a difficult problem heretofore to obtain iron concentrates with low impurity content by means of magnetic separators since they do not provide the required high selectivity of separation of grainy minerals. Furthermore, it is very important that, apart from high selectivity, separators should have a sufficiently high ore throughput, for example 10 to 15 tons per hour.
It is known that in order to accomplish selective separation of grainy minerals the hydrodynamic conditions of pulp flow in a separator must have the following characteristics: high turbulence of the pulp in the feeding device for mineral grains to be disintegrated and partially cleaned before the pulp passes into the separator housing; a tranquil upward helical flow in the middle and top parts of the housing where separation of grainy minerals is accomplished in the main, this being aimed at avoiding vertical pulp flow circulation which causes stirring of grainy minerals in this area and hampers the separation process; partial pulp circulation and wash water counterflow against settling grains of magnetic minerals; and a pulp travel path which prevents the pulp feed flow from getting directly into the bottom part of the housing where the magnetic product is discharged.